Surface cleaning and repair of the surface of, for example, buildings, vehicles, and energy collection devices, are time-consuming and costly, and a surface with an inherent repellency of water, oil, and dirt can be a significant advantage. Surface wetting is governed by surface-energy parameters between the surface and the contacting liquid or solid surface. Where the sum of the free surface energies of the contacting materials components is very low, adhesion between these materials is weak. Hence, it is generally beneficial to lower the free surface energy of an edifice if one hopes to ignore its cleaning and repair. Non-stick materials, such as perfluorinated hydrocarbons have very low surface energies, and few materials adhere to Teflon®. The wetting of these low surface energy materials is reflected in the contact area that is observed between the surface of the low surface energy solid and a wetting material. The interactions between these materials generally result from van der Waals forces.
Nature diminishes the interaction of a surface of a solid and water without resorting to materials with surface energies as low as Teflon®. This is achieved by reducing the amount of the surface that contacts the water. For example, lotus leaves, cabbage leaves, and various fruits are covered by small wax bumps that reduce the van der Waals contact area presented to a water droplet, which forms due to its high surface tension, and significantly reduces the adhesion of the droplets to the surface. A superhydrophobic textured surface displays a water contact angle that exceeds 150° and displays a low sliding angle, which is the critical angle from horizontal of the inclined surface where a water droplet of a defined mass rolls off the inclined surface. This “Lotus effect” provides a self-cleaning surface, as contact water droplets adhere to dust particles and, to a much lesser degree, to some oils that are poorly adhered to the surface, to allow the “dirt” to be carried away as the water droplet rolls from the surface. Most oils are not readily removed from hydrophobic surfaces as the enlarged surface area increases the effective van der Waals interface and the Lotus-effect surface does not repel oils that interact less favorably with water than with the superhydrophobic surface.
Oil repellent surfaces are an engineering challenge because the surface tensions of oily liquids are usually in the range of 20-30 mN/m. The essential criterion for having a surface with superoleophobicity is to maintain oil drops in a Cassie-Baxter (CB) state, one where vapor pockets are trapped underneath the liquid. The CB state is dependent on the surface's structure and the surface energy of the material. If the structure and surface area are insufficient, the meta-stable energetic state is transformed into a Wenzel state, which displays wetting of the structure. The geometric structures that allow a CB state have re-entrant features, such as mushroom heads, micro-hoodoos, or horizontally aligned cylindrical rods. A re-entrant structure implies that a line drawn vertically, from the base solid surface through the geometric feature, must proceed through more than one solid gas interface of that feature.
One problem with these superhydrophobic or superoleophobic structures is a lack of durability. To this end, a material that has a long life when exposed to the environment without loss of the shape and surface functionality is desired, because durability is critical for successful implementation of a superhydrophobic or superoleophobic application.